1. Field of the Invention
The present disclosure relates generally to positioning protocols, and more specifically to techniques for providing improved reliability in positioning protocols using non-piggybacked acknowledgements.
2. Related Art
The Long Term Evolution (LTE) standards for wireless communication are being developed by the 3rd Generation Partnership Project (3GPP). In the 3GPP TS 36.355 specification, which defines the LTE Positioning Protocol (LPP), the LPP Reliable Transport requirements for the transport layers are defined. LPP Reliable Transport specifications include requirements for duplicate message detection, acknowledgement, and retransmission of messages. The LPP specifications require that the User Equipment (UE) support LPP Reliable Transport.
The acknowledgement procedure set out in the TS 36.355 specification includes the following stages:                Upon reception of an LPP message which requests acknowledgement, a receiver returns an LPP message with an acknowledgement response that includes the sequence number of the message to be acknowledged.        An acknowledgement response may contain no LPP message body (also referred to herein as a “non-piggybacked LPP acknowledgement”). Alternatively, the acknowledgement may be sent in an LPP message along with an LPP message body (also referred to herein as a “piggybacked LPP acknowledgement”).        Once the sender receives an acknowledgement for an LPP message, and provided any included sequence number is matching, the sender is permitted to send the next LPP message.        When an LPP message which requires acknowledgement is sent and not acknowledged, the LPP message is resent up to three times by the sender following a timeout period. If still unacknowledged after that, the sender aborts all LPP activity for the associated session.        
The sender of the LPP message requiring acknowledgement enters a “wait for acknowledgement” state while waiting for the acknowledgement to be received in which the sender cannot transmit subsequent messages that include an LPP message body. When the acknowledgement is sent by the receiver of the message, but is lost in transit to the sender, the sender can become stuck in the wait-for-acknowledgement state until the timeout occurs.